High-frequency oven



April 29, 1969 G. ZWANENBURG HIGH-FREQUENCY OVEN Original Filed June 9, 1966 INVENTOR. GOOITZEN ZWANENBURG AGEN United States Patent 3,441,876 HIGH-FREQUENCY OVEN Gooitzen Zwanenburg, Emmasingel, Netherlands, assignor, by mesne assignments, to US. Philips Corporation, New York, N.Y., a corporation of Delaware Continuation of application Ser. No. 556,378, June 9, 1966. This application Apr. 15, 1968, Ser. No. 721,514 Claims priority, application Netherlands, July 2, 1965, 6508506 Int. Cl. H03b ]/02 11.5. Cl. 331-483 8 Claims ABSTRACT OF THE DISCLOSURE An electron tube oscillator for use with a high frequency heating aparatus comprises a pair of serially connected electron tubes connected in the control grid circuit of the oscillator tube. One of the tubes is controlled by a control voltage derived from the oscillator load circuit, and the other is controlled by a separate control voltage derived from the anode current of the oscillator tube. An increase of the latter control voltage causes a decrease in the impedance of the corresponding electron tube.

This application is a continuation of application Ser. No. 556,378 filed June 9, 1966, and now abandoned.

This invention relates to high-frequency ovens having a back coupled grid-controlled electron tube oscillator, wherein the control gird D.C.-circuit of the electron tube oscillator includes a grid-controlled electron tube, con nected as a variable resistor between the control grid and the cathode of the back coupled oscillator tube, said gridcontrolled electron tube being controlled by a control voltage derived from the load circuit.

The grid-controlled electron tube connected as a variable resistor between the control grid and the cathode of the oscillator tube may advantageously be formed by a pentode in order to obtain a sensitive control, for example, tor accurate stabilization of the voltage or current or the temperature of the workpiece such as desired, for example, for the purification of germanium or silicon.

However, when using such a high-frequency oven, especially of high power, it has been found that for satisfactory operation the magnitude of the output must lie within comparatively narrow limits. Thus, for example, extensive experiments with a 25 kw. high-frequency oven have shown that the magnitude of the output can be varied only between 25 kw. and kw.

An object of the invention is to provide a high-freq-uency oven of the kind mentioned in the premeable in which, in addition to a very sensitive control, considerable widening of the limits within which the output may be varied, for example, between kw. and 10 kw., is obtained in a simple manner.

The arrangement according to the invention is characterized in that the control grid D.C. circuit of the oscillator tube includes, in series with the grid-controlled electron tube connected as a variable resistor, an adjusting tube controlled by a separate cotnrol voltage. The impedance from which the separate control voltage for the adjusting tube is derievd is connected in series with the back coupled oscillator tube. An increase in the control voltage produces a decrease in the resistance formed by the adjusting tube.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, With reference to the accompanying diagrammatic drawing.

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In the illustrated high-frequency oven according to the invention, the high-frequency energy required for heating purposes is derived from a grid-controlled electron tube oscillator having a class C connected triode 1. The anode circuit of the tube oscillator, which is designed as a colpitts circuit, includes an oscillatory circuit 2 that determines the oscillation frequency. The circuit comprises a coil 3' shunted by two series-connected capacitors 4 and 4'. The junction of the capacitors is connected to the grounded cathode of triode 1. One end of the oscillatory circuit 2 is connected, via a blocking capacitor 5, to the anode of tube 1, and the other end is connected, via a grid capacitor 6, to the control grid thereof.

The anode of tube 1 is energized through a high-frequency choke 7, from a DC. supply source 8, shunted by a high-frequency decoupling capacitor 9.

The oscillatory current which occurs in the oscillatory circuit 2 during operation is used for heating a workpiece 10 included in a load circuit 12. The load circuit inclcdes a heating coil 11. To this end, the heating coil 11, which is connected to ground at one end, is connected to the output terminals of a coupling coil 13, which in turn coupled by inductive means to the coil 3. The inductive coupling between the coil 3 and the coupling coil 13 is variable to permit load matching.

In order to control the grid excitation of the triode 1, the control grid D.C. circuit includes a variable resistor in the form of a pentode 14. The pentode tube is shunted by a resistor 15 to prevent the grid resistance of triode 1 from exceeding a predetermined permissible limit. The screen grid voltage of the pentode 14 is derived from a separate supply voltage source 16. The control grid D.C. circuit also includes a high-frequency decoupling network comprising a series coil 17 and a by-pass capacitor 18.

In the arrangement described, the control of the grid excitation of triode 1 is used for stabilizing the voltage in the load circuit. To this end, a control voltage derived from the load circuit 12 is applied through a control voltage circuit 19, to the control grid of pentode 14. In its practical form, the control-voltage circuit 19 includes an input network comprising blocking capacitors 20, 20' and a high-frequency choke 21 in between, followed by a rectifying device 22 and an associated output resistor 23. The output resistor 23 is connected to the input terminals of a direct voltage amplifier 24, via a source of constant bias constituted by a gas-filled tube 25 which is connected via a series-resistor 26 to a supply voltage source 27. An input voltage equal to the output voltage from the rectifying device 22 minus the operating voltage of the gas-filled tube 25 is thus set up at the input terminals of the direct voltage amplifier 24. This input voltage is applied, after amplification in the direct voltage amplifier 24, as a control voltage of negative polarity to the control grid of pentode 14.

The arrangement so far described provides accurate stabilization of the voltage in the load circuit 12. More particularly, if the voltage in the load circuit 12 increases, the negative control voltage will increase. The grid resistance of triode 1, which is formed by the pentode 14, also increases, resulting in a shift of the grid bias of triode 1 and a corresponding decrease in grid excitation which counteracts the increase of voltage in the load circuit 12. Conversely, upon a decrease of voltage in the load circuit 12, an increase in grid excitation will occur due to reduction of the grid resistance of triode 1. The increased grid excitation counteracts the decrease of voltage in the load circuit 12.

A sensitive control of the voltage in the load circuit 12 to a predetermined value is thus obtained which may be made adjustable, if desired, by utilizing a voltage divider as the output resistor 23 of the rectifying device 22.

However, in the event of a large variation in load in the output circuit of the oscillator tube 1, resulting in a large variation in output, the control described has been found not to come up to expectations. Notably, the variations in voltage or current in the control grid D.C.-circuit, caused by the variations in load, may result in an adjustment of the pentode 14 outside its working range and this may render the complete control inoperative, or even cause a defect of the pentode 14.

According to the invention, in addition to a sensitive control, I obtain in a simple manner a considerable widening of the limits within which the magnitude of the output may be varied. This is achieved by including in the control grid D.C.-circuit of the oscillator tube 1, in series with the pentode 14 connected as a variable resistor, an adjusting tube 28 controlled by a separate control voltage. The circuit further includes, in series with the back coupled oscillator tube 1, an impedance from which the separate control voltage for the adjusting tube 28 is derived. An increase in said control voltage produces a decrease in the resistance formed by the adjusting tube 28 In the embodiment described, the series impedance is included in the form of a series resistor 29 in the D.C. supply circuit of the oscillator tube 1. Resistor 29 is connected between the grounded cathode of the oscillator tube 1 and the negative terminal of the D.C. supply source 8. The direct voltage set up across the series resistor 29 is applied, after conversion in a D.C.-A.C. voltage converter (chopper) 30, through a transformer 31, to a full wave bridge rectifying device 32 having an output impedance 33 for producing the control voltage for the adjusting tube 28. The control voltage is applied with positive polarity to the control gr-id thereof. Because of different direct voltage levels of the cathode circuits of the oscillator tube 1 and the adjusting tube 28, the isolation obtained between said circuits by means of the transformer 31 is of great practical advantage.

A description of the operation of the specified arrangement now follows:

If the load on the oscillator increases the oscillator current will increase and the output voltage will decrease, resulting in a decrease in the grid excitation and the negative grid bias of the oscillator tube 1. In this case, the voltage increase across the series resistor 29 caused by the increase in direct current of the oscillator will bring about, after conversion and possible amplification in the D.C.-A.C. voltage converter 30, and subsequent rectifica tion in the rectifying device 32, an increase in the positive control voltage at the control grid of the adjusting tube 28. This produces a decrease in the resistance formed by the adjusting tube 28, so that the decrease in negative grid bias of the oscillator tube 1 is accompanied by a decrease in the resistance, formed by the adjusting tube 28, in the control grid D.C.-circuit of the oscillator tube 1. Conversely, upon a decrease in load on the oscillator tube 1, the resulting increase in negative grid bias of the oscillator tube 1 will be accompanied by an increase in the resistance of the adjusting tube 28.

The variations in negative grid bias of the oscillator tube 1, which occur due to variations in load on the oscillator, are eliminated substantially by variations in the resistance formed by adjusting tube 28. As a result, the adjustment of the pentode 14 within its working range is retained independently of the load on the oscillator, the pentode 14 thus causing accurate stabilization of the voltage in the load circuit 12 in the manner described hereinbefore.

No interfering interaction occurs between the two control systems formed by the series-connected adjusting tube 28 and pentode 14, which are controlled by separate control voltages, even if operating conditions vary greatly. More particularly, no interfering instabilities occur because the two control systems 28 and 14 are found to support each other. Thus, not only are the limits for varying the output of the oscillator widened to, for example, between 25 kw. and 10 kw. but this widening is also accompanied by an extremely sensitive stabilization of voltage in the load circuit 12 of the oscillator. The voltage variations are decreased by a factor of, for example, 30 throughout the load region of the oscillator. Due to the stabilizing action of the adjusting tube 28 on the Working range' of tube 14 it is even made possible, in order to increase further the sensitvity of the control, to use a more sensitive type of tube for the tube 14. As is usually the case, these tubes have only a small control region, for example, pentodes of high mutual conductance, triodes of high internal resistance, or the like.

The data for a 25 kw. high-frequency oven which has been extensively tested in practice follows:

Triode 1-TBW 12/25 Pentode 14--EL 34 (6X) Triode 28TB 4/ 1250 Resistor 29--6.8 ohms It should be noted that, in addition to the embodiment described in detail hereinbefore, further embodiments are possible within the scope of the invention. For example, to control the adjusting tube 28, instead of using the D.C. anode current of the tube oscillator by including the series-resistor 29 in the D.C. supply circuit of the oscillator tube 1, it is possible to use the A.C. anode current of the tube oscillator by including a series-impedance in the anode A.C.-circuit of the oscillator tube 1, since the magnitude of the A.C. anode current of the tube oscillator will vary with the load in exactly the same manner as the D.C. anode current. In this case, once again it is advantageous to arrange an isolation transformer between the series impedance and the adjusting tube 28. The alternating voltage derived from the separating transformer is applied in the manner shown, after rectification in a rectifier, as a control voltage of positive polarity to the control grid of adjusting tube 28. However, the embodiment shown, in which the control voltage is derived from a series-resistor included in the D.C. circuit of oscillator tube 1, affords the practical advantage that a low-frequency transformer can be used as the isolation transformer 31.

It is further to be noted that the tube 14 may also be used for stabilizing the current in the load circuit 12 or the temperature of the workpiece 10. In the latter case a thermocouple, for example, is connected to the Workpiece and produces an output voltage which, possibly after amplification, is applied to the pentode 14 through the gas-filled tube 25 and the direct voltage amplifier 24.

What is claimed is:

1. A stabilized electron tube oscillator circuit for energizing the load circuit of a high frequency heating apparatus comprising, a first grid controlled electron tube connected to oscillate at a high frequency, second and third electron tubes each having a control grid, means connecting said second and third tubes in series between the control grid and the cathode of said first tube, means for applying a control voltage derived from said load circuit to the control grid of said second tube thereby to control the current flow therein as a function of the load circuit voltage, an impedance element coupled to said first electron tube for producing a second separate control voltage that is determined by the anode current of said first tube, and means for coupling said second control voltage to the control grid of said third tube thereby to control the current flow therein as a function of said anode current, an increase in said second control voltage being operative to effectively decrease the impedance of said third tube.

2. An oscillator circuit as claimed in claim 1 wherein said impedance element comprises a resistor connected in bseries with the anode-cathode path of said first electron tu e.

3. High frequency heating apparatus comprising, a back coupled grid controlled electron tube oscillator, a grid controlled electron tube directly connected as a variable resistor between the control grid and the cathode of the back coupled oscillator tube, means for controlling said grid controlled electron tube by means of a control voltage derived from the load circuit, an adjusting tube connected in series with the grid controlled electron tube in the control grid D.C. circuit of said oscillator tube, means for controlling said adjusting tube by means of a separate control voltage, an impedance for deriving the separate control voltage for the adjusting tube, and means connecting said impedance in series with the back coupled oscillator tube, whereby an increase in said separate control voltage causes a decrease in the resistance formed by the adjusting tube.

4. A heating apparatus as claimed in claim 3 further comprising a transformer intercoupled between said impedance and the adjusting tube, means for supplying to said transformer an alternating voltage derived from said series impedance, a rectifier, means for applying the alternating voltage derived from the transformer to said rectifier to produce a rectified control voltage of positive polarity, and means for applying said control voltage to the control grid of the adjusting tube.

5. A heating apparatus as claimed in claim 3 wherein said series impedance is a resistor.

6. A heating apparatus as claimed in claim 5 further comprising a DC supply source,.means connecting the cathode of the oscillator tube to ground, and means connecting the series resistor in the DC. supply circuit of the oscillator tube between the grounded cathode of the oscillator tube and the negative terminal of the DC. supply source,

7. A heating apparatus as claimed in claim 4, further comprising a D.C.-A.C. voltage converter connected between the series impedance and the' transformer input.

8. A heating apparatus as claimed in claim 3 wherein said adjusting tube comprises a triode and said grid-controlled electron tube comprises a pentode.

No references cited.

JOHN KOMINSKI, Primary Examiner.

US. Cl. X.R. 

